设x1和x2为任意实数且x2>x1,则
y2-y1==x2^3-x1^3=(x2-x1)(x1^2+x2^2+x1x2)=(x2-x1)[(x1^2+x1x2+1/4x2^2)+3/4x2^2]
=(x2-x1)[(x1+1/2x2)^2+3/4x2^2]
因为x1